Monolithically integrated semiconductor circuits have the ability to provide large currents to a load. These load currents must be accurately monitored to ensure that the proper current magnitude is being supplied, thereby correctly driving the load and/or preventing damage. A power amplifier driving a tape drive motor is an example of such an application. The tape drive motor typically requires a large current wherein the magnitude must be known at any time. As tape is moved from one reel to another, a tension develops across the tape resulting from a differential torque between the two reels. The tape tension must be great enough to properly read data therefrom as it passes over a tape head, but not so great as to stretch or break the tape. Consequently, the motor current must be known to accurately control the differential torque.
A design goal of a power amplifier is to present the least resistance possible in those devices in series with the load. Obviously, if a load current is large, even a small resistance in series therewith will develop a substantial voltage drop. This in turn adversely affects the power amplifier efficiency. Vertical metal oxide semiconductor (VMOS) technology and lateral drain metal oxide semiconductor (LDMOS) technology are examples of a technologies employing devices capable of supplying large currents while having very low transistor channel resistances. In LDMOS technology, for example, a NMOS transistor's gate is driven to a voltage magnitude substantially higher than the voltage magnitude at its drain by a charge pump circuit. As a result, the transistor operates in a deep linear region such that its R.sub.dson (resistance from source to drain with the channel turned on) is minimized. This provides a very efficient transfer of load current to the motor or other load.
In order to provide feedback for properly driving the load, the load current must be monitored. Several problems are encountered in accurately monitoring the load current. The load current is large, and is switched on and off at high frequencies. Tape drive motors present this type of load current measurement problem. The tape drive motor is typically controlled by pulse width modulating (PWM) control signals for pulsing the load pulse load voltage to maintain an average current at high frequencies. The torque applied to the motor, and hence the tension developed across the tape as it passes over a tape head, is critical for reliably reading the data from the tape. The load current must, therefore, be accurately monitored so that the pulse width may be adjusted accordingly.
A simple method for monitoring the load current involves inserting a current sense device in series with the load to measure the load current. This method has at least two drawbacks. The current sense device introduces additional resistance in series with the load current, and hence, lowers the power amplifier efficiency. Furthermore, the load current tracking accuracy is limited by the current sense device accuracy which is susceptible to many semiconductor processing variations. Pepper, in U.S. Pat. No. 4,713,607, describes a current sensing scheme for determining whether a current exceeds a predetermined level. A conductive trace, having a predetermined resistance on an etched circuit board, is connected in series with the load. A comparator is coupled to the conductive trace for comparing a voltage developed at a first point on the trace to a reference voltage at a second point on the trace. The conductive trace, therefore, is the sense element. This scheme does not continuously monitor the load current but merely checks for a predetermined current magnitude. Pepper's invention relies on a sense element and comparator that are not integrated on the same semiconductor substrate as the power devices.
A similar technique is taught by Schmerda, et al., in U.S. Pat. No. 4,945,445 wherein a wire bond is used as the sense element which is in series with the load for sensing load current. Two points separated by a predetermined distance on the wire bond are coupled to a comparator for determining the voltage differential therebetween and hence the current. This invention has the advantage of not introducing additional resistance in series with the load. Like Pepper, the sense element and associated signal processing circuitry is not integrated on the same substrate as the power amplifier. Furthermore, the sense element here acts as a fuse such that load current is not continuously sensed. A technique using an off chip sense resistor and signal processing circuitry for monitoring the current sourced in a power supply is described by Ishii in U.S. Pat. No. 4,860,153.
Another sense resistor technique is taught by Yundt in U.S. Pat. No. 4,804,903. Yundt describes a monitoring apparatus using two operational amplifiers isolated from the power amplifier by coupling capacitors. This circuit is unable to operate at a 100% duty cycle, and is unsuitable for direct current or high frequency applications.
All current sensing techniques described above have an inherent inaccuracy effect resulting from the sensing element and the signal processing circuitry being separate components from the power amplifier. One aspect of this inaccuracy, for example, results from differing environmental conditions of the sense element and the power amplifier.
Load current monitoring accuracy can be improved by integrating the sense element and signal processing circuitry on the same semiconductor substrate as the power amplifier or switching transistors. Vajdic, et al., describe, in U.S. Pat. No. 4,727,309, a current source which is stable over operating condition changes wherein two current sources vary similarly over the span of operating conditions. Another integrated approach is set forth by Chieli in U.S. Pat. No. 4,827,207, wherein a load current flowing through a first switching device is mirrored into a second switching device. A sense voltage at the first device is duplicated at the second device. An operational amplifier then process the duplicated sense voltage information.
In U.S. Pat. No. 4,553,084, Wrathall describes a current sensing circuit incorporating a pilot sense transistor having a sensing resistor in its source leg. The pilot sense transistor is a portion of a large switching transistor and so variations in the large switching transistor can be accurately tracked by the pilot sense transistor. An operational amplifier monitors the signal provided by the sensing resistor for providing feedback information to a driver circuit. Wrathall describes another apparatus for sensing load current in U.S. Pat. No. 4,820,968 which uses a sense resistor in the leg of a current mirroring transistor to convert a mirrored current into a voltage. This voltage is then compared to a reference voltage generated by a reference current which is a equal to a portion of the load current.
A problem associated with the integrated current monitoring inventions described above is with monitoring load currents having high frequency switching characteristics, for example, high frequency PWM. This problem arises because power amplifier technology is necessarily optimized for efficient power or switching transistors. As a result, the integrated operational amplifiers suffer in speed and accuracy.
Thus what is needed is a load current monitoring circuit for accurately generating a sense current that is a fraction of the load current wherein the monitoring circuit is integrated onto the same substrate as the power or switching transistors and is capable of operating at high frequencies.